In computer networks, such as the Internet, preventing a server from becoming overloaded with requests from clients may be accomplished by providing several servers having redundant capabilities and managing the distribution of client requests among the servers through a process known as “load balancing.”
In one early implementation of load balancing, a Domain Naming System (DNS) server connected to the Internet is configured to maintain several IP addresses for a single domain name, with each address corresponding to one of several servers having redundant capabilities. The DNS server receives a request for address translation and responds by returning the list of server addresses from which the client chooses one address at random to connect to. Alternatively, the DNS server returns a single address chosen either at random or in a round-robin fashion, or actively monitors each of the servers and returns a single address based on server load and availability.
More recently, a device known as a “load balancer,” such as the Web Server Director, commercially available from the Applicant/assignee, has been used to balance server loads as follows. The load balancer is provided as a gateway to several redundant servers typically situated in a single geographical location referred to as a “server farm” or “server cluster.” DNS servers store the IP address of the load balancer rather than the IP addresses of the servers to which the load balancer is connected. The load balancer's address is referred to as a “virtual IP address” in that it masks the addresses of the servers to which it is connected. Client requests are addressed to the virtual IP address of the load balancer which then sends the request to a server based on server load and availability or using other known techniques.
Just as redundant servers in combination with a load balancer may be used to prevent server overload, redundant server farms may be used to reroute client requests received at a first load balancer/server farm to a second load balancer/server farm where none of the servers in the first server farm are available to tend to the request.
One rerouting method currently being used involves sending an HTTP redirect message from the first load balancer/server farm to the client instructing the client to reroute the request to the second load balancer/server farm indicated in the redirect message. This method of load balancing is disadvantageous in that it can only be employed in response to HTTP requests, and not for other types of requests such as FTP requests. Another rerouting method involves configuring the first load balancer to act as a DNS server. Upon receiving a DNS request, the first load balancer simply returns the virtual IP address of the second load balancer. This method of load balancing is disadvantageous in that it can only be employed in response to DNS requests where there is no guarantee that the request will come to the first load balancer since the request does not come directly from the client, and where subsequent requests to intermediate DNS servers may result in a previously cached response being returned with a virtual IP address of a load balancer that is no longer available.
When redundant server farms are situated in more than one geographical location, the geographical location of a client may be considered when determining the load balancer to which the client's requests should be routed, in addition to employing conventional load balancing techniques. However, routing client requests to the geographically nearest server, load balancer, or server farm might not necessarily provide the client with the best service if, for example, routing the request to a geographically more distant location would otherwise result in reduced latency, fewer hops, or provide more processing capacity at the server.